1. Field of the Invention
This invention relates generally to semiconductors and fabrication methods thereof and, more particularly, to a semiconductor device and fabrication method thereof in which an electrical characteristic of a transistor can be changed to a desired characteristic in a non-volatile memory device.
2. Description of the Related Art
In recent years, semiconductor devices (integrated circuits (IC)) having an analog or a digital circuit have been used in various types of electronic devices such as home electric appliances, communication devices, and the like, and have become a key part of progress in the electronics industry. In the constantly advancing field of electronics, an IC having a new circuit is being developed and manufactured everyday. For the development of a new IC, circuit simulation or the like is first carried out and then the circuit is designed so as to obtain the desired characteristics identified during the circuit simulation. Next, a mask, such as a reticle or the like, is produced and a prototype of the IC is manufactured in a process factory. When the prototype of the IC satisfies desired electrical characteristic values, manufacture starts as the next step in the development sequence.
More particularly, flash memories are widely used as non-volatile memories. There are flash memories having an ONO (Oxide/Nitride/Oxide) film such as MONOS (Metal Oxide Nitride Oxide Silicon) films or SONOS (silicon Oxide Nitride Oxide Silicon) films. This type of flash memory has a silicon nitride film, also known as a trap layer, which is provided for storing the charge, the silicon nitride film being sandwiched between silicon oxide films. An example of a flash memory having an ONO film is disclosed in U.S. Pat. No. 6,011,725. In such ONO film flash memories, data is programmed by storing the charge in the silicon nitride film. Such stored charge changes the threshold voltage of the transistor in a non-volatile manner. Reading the data is accomplished by reading the threshold voltage. Erasing the data is accomplished by extracting the stored charge.
In conventional IC development, if the prototype of the IC does not satisfy the desired electrical characteristic value, the reticle pattern has to be redesigned, a new reticle or the like has to be produced, and the prototype IC manufactured again. In cases where the aforementioned feedback is necessary, the burden of managing resources of time and cost for IC development and fabrication is increased. In particular, managing resources for the IC development can be problematic when the custom IC is produced for a small business customer that demands only a small number of final ICs in total. In addition, circuit simulators for the simulation of analog circuits are not as accurate as that for digital circuits. For this reason, a prototype is sometimes fabricated several times to consider fluctuations in the manufacturing process such as the dimension of a gate length, ion implantation, and other manufacturing parameters.
For example, after the product is fabricated on a wafer or chip, the interconnection can be cut to obtain the desired electrical characteristic value for the circuit. However, it takes time to cut the interconnection and a laser or the like is necessary for cutting the interconnection. Also, cutting the interconnection is destructive and irreversible. If the circuit characteristic is changed too much, the electrical characteristics cannot be recovered.